Many different kinds of rock bolts are used to secure rock strata underground. One kind comprises a length of reinforcing bar having ribs along its length. The ribs enhance the anchoring ability of the bolt. In the case where a resin with a catalyst is used to secure the bolt in a hole in the rock matrix, the bolt is rotated in the hole to mix the resin and the ribs also serve the purpose of assisting the mixing of the resin. However, known ribbed formations on rock bolts do not provide good mixing characteristics.
The end of the bolt which is intended to project from the rock face has a rolled thread thereon which receives a nut used to tension the bolt. The bar from which such a prior art rock bolt is made is manufactured in a steel mill and is supplied with the ribs formed on the whole length of the bar. Thus, in order to provide the rolled thread on the bolt, the bolt has to be machined to remove the ribs on a section thereof and to ensure that such section has a circular cross-section. Such a machining operation adds to the cost of the prior art rock bolt.
U.S. Pat. No. 4,584,247, to Mulholland proposes a way of overcoming the abovementioned problem. Mulholland teaches a method of providing a threaded portion on a hot rolled steel bar having chevron patterned ribs thereon. The method involves selecting a ribbed bar having an overall average cross-sectional area substantially the same as the average cross-sectional area that is desired for the threaded section of the bar. Such a bar is inserted on to a thread rolling machine and the threaded section is roll formed on the bar. The ribs are not removed before the threaded section is formed. In other words the thread in the Mulholland teaching is rolled onto the bar despite the presence of the ribs thereon. This method has substantial disadvantages and cannot be put into effective practice. Because of the presence of the ribs the bar is not circular in cross-section which means that the thread formed thereon cannot be circular. This arises from the fact that from place to place on the bar there is material "missing" from the thread. Such a thread cannot meet standard thread specifications and is not appropriate for the high tensile requirements for rock bolts in underground mining operations. Furthermore, the ribs of the bar according to the Mulholland teaching can at best only have a very slight depth. If the depth of the ribs is increased it will become a matter of impossibility to roll a thread on the bar which is not totally defective. Because the ribs can only have a slight depth the bar suffers from a poor anchoring ability, for it is primarily the ribs on a bar of this kind which give it its anchoring ability. A further consequence of a bar with ribbing of a slight depth is that it provides poor mixing in the case of resin anchoring of the bar.
It is an object of the invention to lessen the problems associated with prior art rock bolts. In this specification, the term "pitch diameter", in relation to a straight thread, means the diameter of an imaginary co-axial cylinder, the surface of which passes through the thread profiles at such points as to make the width of the groove equal to one half of the basic pitch of the thread. On a perfect thread this occurs at the point where the width of the thread and groove are equal.
The term "bar" when used in this specification includes a pipe and a bar with an axial bore.